The soot generated by diesel engines must be removed from the exhaust gas by means of soot filters in order to prevent air pollution. With increasing deposition of soot in the filter, the exhaust-gas back pressure generated by the filter increases and reduces the power of the engine. The filter must therefore be regenerated regularly by burning off the soot.
The regeneration poses a problem in particular in modern diesel engines, since the exhaust gas of said engines in normal operation is too cold to ignite the combustion of the soot on the filter. Exhaust-gas temperatures of 500 to 700° C. would be necessary for this purpose.
Passive and active methods for the regeneration of soot filters are known. In the passive method, for example according to EP 0 341 832 A2, a soot filter is oxidized continuously at exhaust-gas temperatures of below 400° C. by means of nitrogen oxide as an oxidant. For this purpose, an oxidation catalytic converter is arranged upstream of the soot filter, which oxidation catalytic converter converts the nitrogen monoxide which is present in the exhaust gas into nitrogen dioxide. Said method requires that the exhaust gas contains sufficient quantities of nitrogen monoxide. It is a disadvantage that the presence of hydrocarbons in the exhaust gas hinders the oxidation of nitrogen monoxide to nitrogen dioxide. This shortcoming can be remedied, according to U.S. Pat. No. 6,877,313 B1, by virtue of two oxidation catalytic converters being arranged upstream of the soot filter, with the first catalytic converter substantially burning the hydrocarbons in the exhaust gas, and the second catalytic converter oxidizing the remaining nitrogen oxides in the exhaust gas to nitrogen dioxide.
Passive methods cannot ensure reliable operation of the soot filter. The filter must be actively regenerated at least from time to time. For this purpose, fuel is injected into the exhaust-gas flow upstream of the oxidation catalytic converter and is burned on the oxidation catalytic converter, or the concentration of unburned hydrocarbons in the exhaust gas is increased by means of engine-internal measures. Depending on the present operating state of the engine, the catalytic combustion in the oxidation catalytic converter must heat the exhaust gas by 200 to 400° C. to reach the soot ignition temperature. Here, high temperature peaks occur in the interior of the oxidation catalytic converter, which temperature peaks can lead to premature ageing of the catalytic converter. As a result, with progressive operating duration, the conversion of the hydrocarbons and of carbon monoxide on the oxidation catalytic converter deteriorates.
DE 10 2005 017 378 A1 describes an exhaust-gas purification device having a catalytic converter body which is composed of a plurality of catalytic converter discs arranged in series in the flow direction of the exhaust gas. The catalytic converter discs are composed of a sintered metal powder for the conversion of the hydrocarbons, carbon monoxide and nitrogen oxides which are contained in the exhaust gas to form carbon dioxide, water and nitrogen. The distance between the catalytic converter discs is approximately 3 mm.
US 2003/0099583 A1 describes an exhaust-gas purification device which comprises a three-way catalytic converter A, for converting hydrocarbons, carbon monoxide and nitrogen oxides in the stoichiometric exhaust gas, and also a catalytic converter B which is arranged downstream and which has zeolites for absorbing hydrocarbons.